Apparatus for polymerizing and forming thin continuous films using a glow discharge



Sept. 26, 1967 a. G. CARBAJAL m, ETAL 3,344,055

. APPARATUS FOR POLYMERIZING AND FCRMING THIN CONTINUOUS FILMS USING A GLOW DISCHARGE Original Filed April 29, 1964 llll lilllll |lllJ l L III TIME !!ll!'!!!l 'fiu/brdiLoY/ ,Jr F .4 v 9 affforny H United States Patent f 3,344,055 I APPARATUS FOR POLYMERIZING AND FORM- ING THIN CONTINUOUS FILMS USING A GLOW DISCHARGE Bernard G. Carbajal IH, and Buford G. Slay, Jr., Richardson, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Original application Apr. 29, 1964, Ser. No. 363,523, now Patent No. 3,318,790, dated May 9, 1967. Divided and this application Feb. 27, 1967, Ser. No. 618,793

3 Claims. (Cl. 204312) ABSTRACT OF THE DISCLOSURE Disclosed are apparatus and methods for forming thin, continuous films in a glow discharge in the region of a perforated electrode. The apparatus includes two electrodes supported ina closed, evacuatable reaction chamber with at least one of the electrodes being perforated, means for supporting a body in the chamber, means for introducing an appropriate volatile mixture into the chamber and electrical means for establishing a glow discharge between the electrodes.

This application is a division of copending application Ser. No. 363,523, filed Apr. 29, 1964, now Patent No. 3,318,790 also assigned to Texas Instruments Incorporated, the assi-gnee of the present application.

This invention relates to apparatus for the formation of thin continuous films, and more particularly to apparatus for the formation of thin, continuous films in a glow discharge in the region of a perforated electrode.

It has been found that the controlled polymerization, or the controlled dissociation and recombination of gaseous mixures in a low pressure glow discharge is a useful method for depositing thin films. In glow discharge polymerization, it has been found that all surfaces in the vicinity of the glow become covered with a thin polymer film. The action is of such nature that materials not commonly considered as polymer precursors, such as napththalene or anthracene, readily form polymers in the glow discharge. Films of oxides, nitrides, or carbides of metals or semimetals are formed by the controlled dissociation and selective recombination of volatile metalor semimetal-containing compounds in a low pressure glow discharge.

The present invention is directed particularly to apparaus for coating a body with a thin uniform, continuous film. A structure forming an evacuated reaction zone has a first conductive electrode mounted therein. A second conductive electrode which is perforated forms a cathode. The second electrode is mounted in the zone parallel to, and in juxtaposition with respect to, the anode. Means are provided for applying a voltage between said electrodes with the cathode electrode of polarity which is negative with respect to the anode. An insulator means supports the body to be coated on the side of said cathode opposite the anode and closely adjacent thereto. Supply means provide an appropriate vaporized reactant mixture within said reaction zone for polymerization and dissociation in the field between the electrodes and for deposit on the body behind the cathode.

For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description .takeri in conjunction with the accompanying drawings in which:

- FIGURE 1 is a sectional view showing one system for carrying out the present invention;

FIGURE 2 is a view taken along line 22 of FIG- URE 1;

FIGURE 3 is a graph illustrating one mode of operation of the unit of FIGURE 1; and

FIGURE 4 illustrates a modification of the invention.

FIGURE 1 illustrates apparatus of the present invention for depositing a thin, continuous film on the surface 11 of the body 10. The reactions are carried out within an evacuated zone in a bell jar 12 which is sealed onto a base plate 13. The body 10 is mounted in holders 14 and 15 which are supported by suitable means (not shown) electrically insulated from the base plate 13.

A pair of electrodes are positioned in jar 12. An anode 16 is mounted on an insulator 17. Anode 16 in this embodiment is a sheet of aluminum which may be two inches wide, six inches tall and one-fourth inch thick. It is mounted about two inches above the base plate 13. The cathode 18 is an electrically conductive screen, e.g., aluminum, of length and width equal to the length and width of the anode 16. The cathode 18 is supported by an insulator 19. The anode 16 and cathode 18 are mounted facing one another and spaced apart about three quarters of an inch.

Conductors 20 and 21 connect the electrodes 16 and 18, respectively, to the positive and negative terminals of a high unidirectional voltage source 22. By applying a high voltage to the electrode 16 and 18, a glow discharge is produced therebetween.

The body 10 may be either a conductor or an insulator. As shown in FIGURE 2, the body 10 has dimensions smaller than the dimensions of the cathode 18 and is positioned parallel to and adjacent the side of the cathode 18 opposite the anode 16. By using a screen electrode for the cathode, the film deposited on body 10 is a thin uniform film. In accordance with the invention, an evacuating pump 25 is connected by way of valve 26 and a suitable cold trap 27 to an exhaust line 28 leading through the base 13. A thermocouple pressure gauge 30 is mounted on the base of the bell jar with a cable 31 passing through the base 13 to a control unit 32. The pressure gauge 30 includes a heat source and a temperature sensing element. Heating current is applied by way of conductors 33 to a heater element in unit 30. The heat transfer to a thermocouple element in unit 30 will then depend upon the pressure inside the bell jar 12. The temperature dependent signal is then applied by way of conductors 34 to the control unit. The control unit 32 controls the evacuating pump 25 by means of channel 35.

A storage tank 38 containing source material is connected by way of valve 39 to an inlet port 40 leading into the bell chamber.

In using the system illustrated in FIGURES 1 and 2 for polymerization of liquid samples in the glow discharge, the vacuum pump 25 is energized until the system is reduced to its ultimate pressure of less than 10- Torr. Organic vapor is then introduced by opening valve 39 until the desired pressure rise is obtained. The pressure rise normally would be from an initial level up to a reaction pressure of about 0.3 Torr. The system is then allowed to stabilize at the desired pressure for about ten or fifteen minutes before the voltage from source 22 is applied to the electrodes 16 and 18 to initiate the glow discharge.

The turn-on voltage, or voltage at which the glow is initiated is dependent upon the pressure, geometry of the system and the material to be polymerized or dissociated. For a given system, the turn-on voltage is character istic for each material. The voltage for the DC. glow is set at a predetermined value above the turn-on voltage and the glow discharge process is then allowed to continue for either a predetermined time or until the glow turns itself off by depletion of the coating material within the jar.

Typical voltages and pressures are recorded in Table I for various compounds suitable for polymer coating purposes.

Conductivity measurements made on some of the polymer films produced in accordance with the invention are recorded in Table II. The values are only approximate due to the difficulty in accurately measuring resistivity in the range of interest. They refer to the minimum bulk resistivity for a polymer film.

TABLE II Polymer precursor: Resistivity, ohm-cm. Ferrocene X Naphthalene 8 x 10 Anthracene 5 X 10 With the exception of ferrocene and vinylferrocene, all of the compounds in Tabe I will form protective films on the cathode. With the system set at a given voltage, the deposit of the protective film will cause the glow to decay or die. A typical current vs. time relationship is plotted in FIGURE 3, where the curve 60 shows that, with the establishment of a glow discharge at time=zero, the current flowing between the electrodes 16 and 18 is relatively high but rapidly decreases to a value approaching zero. Thus, the glow discharge will die out at some time after its initiation. If desired, film thickness may be controlled by removing or disconnecting the voltage source from the electrodes at some time prior to the time that the discharge naturally would be extinguished. For the materials that form a suitable polymer, the film thickness on the cathode is a function of the voltage at which the glow decays or dies. For given geometry, control of the film thickness on a substrate placed behind the screen can be obtained. Film thickness of the order of 0.1 to 1.0 microns have been found to ,be readily achieved.

Many of the polymers have been found to exhibit a photoconduc-tive effect. That is, the conductivity of the film increases by one or more orders of magnitude under the influence of ultraviolet light. The response is markedly greater for short wavelength light than for longer wavelength light. With the exception of the vanadium chelate polymer, the effect appears to be general. The intensity of the photoconductive effect varies depending upon the polymer precursor.

While glow discharge polymerization has heretofore been carried out, it has been found by applicants that the use of'the screen cathode, which casts its shadow onto the body to be coated, permits the formation of a uniform, thin continuous film. In general, the films formed are clear and colorless, exhibiting typical interference colors related to thickness and refractive index. The polymers have a dielectric constant of about 3.2. On smooth surfaces, the polymer films appear to have very uniform surface characteristics and are highly adherent to the body on which they are formed. The polymer films thus produced have been found to withstand repeated cycling from room temperature to liquid helium temperature with no apparent loss of the insulating properties.

The screen 18 should be such that the metal portions are preferably of smaller area than the area of the holes through the cathode. Aluminum screen wire of about 10 mesh has been found to be satisfactory. It provides an open pattern through which the plasma may pass. It appears that the field causes acceleration of the particles through the screen onto the body 10. The result is that an even coating will be deposited on the body, whereas if the body were to be placed between the cathode and anode, an uneven coating would be the result. Of course, it is possible to coat a solid conductor by substituting the conductor for the screen cathode. However, semiconductors and insulators as well as conductors can be given uniform coatings of high quality and of uniform thickness by accelerating the coating material through an open or reticulated cathode.

Voltages and pressures for carrying out the present invention are generally well understood by those skilled in the art, as indicated by the disclosure of Patent No. 3,068,510 to Coleman.

The system illustrated in FIGURE 1 has been described with respect to DC. operation. However, it may also be operated with an A.C. voltage source. FIGURE 4 illustrates an A.C. system in which a pair of screens 61 and 62 are energized from an A.C. source 63 to establish a glow discharge. Bodies 64 and 65 may thus be simultaneously coated. In each case, the electrodes 61 and 62 are screens having areas preferably greater than the area of the bodies 64 and 65. The bodies 64 and 65 are mounted parallel to and adjacent each electrode on the side opposite the other electrode.

It is important to note that by the use of a perforated or screen-like electrode, any type of body can be coated. For example, in complex structures in which conductors overlay insulators, the entire face of such a body can be coated uniformly by depositing the film-forming polymer onto the body after passage through the screen electrode. The invention is particularly useful in applying coatings to insulating bodies or semiconductor bodies having active and passive elements formed therein and interconnected by conductive paths applied to the surface. Such -a body may be supported in an evacuated reaction zone into which an appropriate reactant mixture is introduced. The glow discharge is then established in the zone between two spaced apart electrodes of opposite polarity, both of which are located in front of or to one side of the body. The species formed by the glow discharge then passes through the plane adjacent to the body for deposit on the body. Preferably, the plane isestablished by the application of a potential to a perforated or screen electrode which will perform the dual function of permitting the establishment of a glow discharge and the movement of the resultant species with substantially unimpeded flow from the region between the two electrodes to the location of the body adjacent the screen electrode.

The apparatus of the invention may also be employed in the controlled dissociation and recombination of volatile metal containing compounds to form insulating or refractory films. Nitrides, oxides, and carbides of metals such as aluminum and titanium and of semimetals such as silicon, and the like, are formed by recombination of the appropriate charged particles in the plasma. The resultant products are then deposited on the surface of the substrate 10. The production of the solid materials is controlled by the relative proportions of the reactant and control gases in the system. The remainder of the particles in the system are recombined as gaseous products which are swept from the system through the vacuum aperture 28.

In using the system illustrated in FIGURES 1 and 2 for the decomposition of volatile metal compounds, the reactive gas used may be varied as desired to produce coatings of various dielectric materials. Generally, the reactant gas may be any volatile compound of a metal such as titanium, aluminum or the like, or of semimetals such as silicon, which when dissociated in a glow discharge will produce an oxide, carbide or nitride, or which will do so whencombined with an appropriate control gas. Thus argon or oxygen is used as a control gas for the formation of oxides, and nitrogen or ammonia for the formation of nitrides. Carbides are obtained through the dissociation of volatile organometallic compounds or by the dissociation of a mixture of a volatile inorganic metal compound and an organic or carbon containing compound. Likewise, the pressures and proportions of control gas to reactive gas are varied to provide optimum conditions for the formation of the desired films. The voltage, current, and spacing of the electrodes may also be varied within wide limits to provide a multitude of operating conditions. The following example is illustrative of the many various operating conditions which have successfully been employed to deposit thin insulating films.

A wafer of gallium arsenide was mounted in holders 14 and 15 as shown in FIGURE 1. The system was evacuated and oyxgen and tetraethoxysilane (C H O) Si were then introduced by opening valve 39 until a total dynamic pressure of 70X 10- Torr. was obtained. Using a 60 cycle alternating current power supply in the system shown in FIGURES 1 and 2, a glow discharge was struck between the electrodes 16 and 18. The voltage drop across the electrodes was maintained at 700 volts. Silicon oxide was deposited on the wafer 10 at a rate of 8 x10 A. per hour.

Thus it has been shown that thin, continuous organic polymer coatings and thin, continuous films of dielectric materials can be deposited by the polymerization of organic polymer precursors or by the decomposition of volatile metal compounds respectively by a gaseous plasma produced with a direct current or alternating current glow discharge.

It is to be understood that the above described embodiments of the invention are merely illustrative of its principles. The apparatus shown and described may be modified in order to accommodate complex surfaces to be coated, and may be adapted for the deposition of other solid films. Various other modifications may be devised by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

What is claimed is: e

1. Apparatus for applying a continuous thin fihn to a 5 body, comprising:

(a) a structure forming an evacuatable reaction zone,

(b) two perforate, confronting electrodes positioned in said zone substantially in parallel spaced apart relationship,

(c) means for supporting a body in said zone adjacent and substantially parallel to one of said perforate electrodes on the side opposite the other electrode,

(d) a storage tank,

(e) means for introducing an appropriate volatile mixture from said storage tank into said zone, and

(f) an A.C. voltage source for establishing glow discharge between said electrodes.

2. The apparatus according to claim 1 wherein the perforated electrodes have dimensions larger than the 20 body to be coated.

3. The combination according to claim 1 wherein the perforated electrodes are metallic screens such that the metal portions occupy a surface area less than the holes between the metal portions.

References Cited UNITED STATES PATENTS 2,799,640 7/ 1957 Pevere et a1. 204-464 3,021,271 2/1962 Wehner 204-192. 3,117,022 1/ 1964 Browson et a1 204192 3,239,368 3/1966 Goodman 204-312 FOREIGN PATENTS 939,275 10/ 1963 Great Britain.

JOHN H. MACK, Primary Examiner.

ROBERT K. MIHALEK, Examiner. 

1. APPARATUS FOR APPLYING A CONINUOUS THIN FILM TO A BODY, COMPRISING: (A) A STRUCTURE FORMING AN EVACUATABLE REACTION ZONE, (B) TOW PERFORATE, CONFRONTING ELECTRODES POSITIONED IN SAID ZONE SUBSTANTIALLY IN PARALLEL SPACED APART RELATIONSHIP, (C) MEANS FOR SUPPORTING A BODY IN SAID ZONE ADJACENT AND SUBSTANTIALLY PARALLEL TO ONE OF SAID PERFORATE ELECTRODES ON THE SIDE OPPOSITE THE OTHER ELECTRODE, (D) A STORAGE TANK, (E) MEANS FOR INTRODUCING AN APPROPRIATE VOLATILE MIXTURE FROM SAID STORAGE TANK INTO SAID ZONE, AND (F) AN A.C. VOLTAGE SOURCE FOR ESTABLISHING GLOW DISCHARGE BETWEEN SAID ELECTRODES. 